1. Field of the Invention
This invention relates to a semiconductor laser device to be suitably used as a light source for optical telecommunications, optical instrumentation, optical data processing and other areas of application. The present invention also relates to a laser module comprising a semiconductor laser device according to the invention and associated components.
2. Prior Art
As is well known, semiconductor laser devices have applications in various technological fields including optical telecommunications, optical instrumentation and optical data processing and massive efforts are currently being paid to develop high-performance devices.
Quantum well type semiconductor laser devices are highly promising because of the low threshold current level they typically have for laser oscillation.
Like other semiconductor laser devices, the quantum well type semiconductor laser device has a threshold current level that varies as a function of temperature. As ambient temperature rises, the device loses its efficiency and its threshold current value. Firstly, this phenomenon will be described on the basis of known theories.
A semiconductor laser device to be used as a light source for optical telecommunications normally emits light with a wavelength in a 1.3 to 1.55 .mu.m band. The temperature dependency of the threshold current level of such a device is approximately expressed by formula (1) below. EQU I.sub.th .varies.exp(T/To) (1)
where T[K] is the temperature of the active layer and To[K] is a characteristic temperature tentatively used to indicate the temperature dependency of the device, which is between 45 and 60K for ordinary use. Thus, with such a relatively small To, a semiconductor laser device that operates in the above wavelength band significantly changes its performance, as it is affected by ambient temperature.
On the other hand, a semiconductor laser device starts oscillating for laser emission when the gain of the resonator exceeds the total loss.
Formula (2) defined below defines the condition of oscillation, where .GAMMA. is the light confinement coefficient of the active layer, G is the gain in the laser resonator and total is the resonator loss, which is expressed by formula (3), where in is the internal loss of the active region and m is the reflectivity loss at the ends of the resonator, which is further expressed by formula (4), where R.sub.1 and R.sub.2 are the reflectivities of the respective ends of the resonator with a cavity length of L. EQU .GAMMA.G=.alpha..sub.total ( 2) EQU .alpha..sub.total =.alpha..sub.in +.alpha..sub.m ( 3) EQU .alpha..sub.m =(1/2.GAMMA.).times.ln (1/R.sub.1 R.sub.2) (4)
FIG. 8 shows a summary of an experiment for determining the device gain G conducted on quantum well type semiconductor laser devices having respective numbers of quantum wells of Nw=4, Nw=6 and Nw=8. Note that temperature is the sole parameter of the measurement.
From FIG. 8, the gain G of the quantum well laser device can be expressed by formula (5) below. EQU G=Go{1.div.ln (J/Jo)} (5)
where Go is the gain coefficient, Jo is the transparent current density and J is the density of the injected electric current density.
FIG. 9 shows the temperature dependency of each of Go and Jo as defined above.
From FIG. 9 it is seen that both Go and Jo degrade with temperature rise.
Thus, from formulas (2) and (3), the threshold current level for laser oscillation of a semiconductor laser device rises with temperature.
FIG. 10 shows a schematic sectional view of a known quantum well type semiconductor laser device with a wavelength band of 1.3 to 1.55 .mu.m.
The semiconductor laser device 1 of FIG. 10 has an active layer comprising a quantum well layer and is provided at an end thereof with a high reflectivity film 3 which is in fact a multilayer structure of a dielectric material in order to decrease the mirror loss .alpha..sub.m. The high reflectivity film 3 shows a reflectivity as high as 95% or more for the entire oscillation wavelength band.
FIG. 11 shows the relationship among the electric current, the optical output and the temperature of a laser module comprising a semiconductor laser device 1 of the above described type as a principle component when it receives an injection signal.
As clearly shown in FIG. 11, the optical output of the module is P.sub.1 at 30.degree. C., whereas the average output level falls to P.sub.2 at 70.degree. C. along with the quenching ratio c expressed by formula (6) below. EQU .gamma.=10 logP.sub.max /P.sub.min ( 6)
As a common practice to avoid this problem, the laser device is normally held to a constant temperature by means of a temperature gauging means such as thermistor and a Peltier device, although the laser module and hence a system comprising such a laser module would become rather costly, because a Peltier device is very expensive.
FIG. 12 shows a schematic block diagram of a laser module comprising a known semiconductor laser device.
Referring to FIG. 12, the laser module 11 comprises a semiconductor laser device provided with a closed loop type drive circuit 14 including an AGC (Automatic Gain Control) circuit 12 and a monitor/optical receiver (photodetector) 13 in such a way that a bias current and a modulation current can be applied to the device 1.
The numeral 11 of FIG. 12 detects monitor light emitted from the back side of the semiconductor laser device 1 by the monitor/optical receiver 13 in order to maintain the average output and the quenching ratio to respective predetermined levels by means of the AGC circuit 12.
[Problem to be Solved by the Invention]
A quantum well type semiconductor laser device (with a wavelength band of 1.3 to 1.55 .mu.m) illustrated in FIG. 10 shows a significant rise in the threshold current for laser oscillation to consequently reduce the quantum efficiency of the device when the ambient temperature rises. It is to be noted that semiconductor laser devices of other types are not free from this problem.
Laser systems comprising such a semiconductor laser device is normally provided with an external circuit such as an APC (Automatic Power Control) circuit in order to minimize the adverse effect of the rise in the ambient temperature. However, such an external circuit can make the system so much more complicated and hence costly. Therefore, it is highly desirable to develop measures to counter the adverse temperature effect on the semiconductor laser device that do not rely on an external circuit.
However, no noticeable technological development has been made in the study of the relationship between the operating temperature and the oscillation wavelength of a semiconductor laser device, trying to provide the device with improved reflectors at the ends of the resonator and optimally exploit the potential of the reflectors so that no external circuit may be required.
A laser module as illustrated in FIG. 12 requires a large and costly control circuit in order to adapt itself to the need of quickly picking up and processing the modulated current.
[Objects of the Invention]
In view of the above described circumstances, it is therefore an object of the invention to provide a semiconductor laser device equipped with improved reflectors at the opposite ends of the resonator and capable of preventing any rise in the laser oscillation threshold current and reduction in the quantum efficiency due to temperature rise. It is another object of the invention to provide a laser module comprising such a semiconductor laser device that operates effectively and efficiently at high temperature.